The present invention relates to a cathode ray tube applied to a color picture tube and the like and, more particularly, to a cathode ray tube including an electron gun assembly capable of dynamic astigmatism compensation.
A self-convergence in-line color picture tube comprises an in-line electron gun assembly for emitting three electron beams in line, i.e., a center beam and a pair of side beams passing through the same horizontal plane, and a deflection yoke for forming a nonuniform magnetic field for deflecting the electron beams emitted by the electron gun assembly. The three electron beams emitted by the electron gun assembly converge on the center of the screen by the action of a main lens portion included in the electron gun assembly, and at the same time self-converge on the entire screen due to a nonuniform magnetic field made up of a pincushion horizontal deflecting magnetic field and a barrel vertical deflecting magnetic field.
As shown in FIG. 1A, electron beams 6 passing through the nonuniform magnetic field are influenced by astigmatism, e.g., forces in the directions of arrows 11H and 11V by a pincushion magnetic field 10. A beam spot 12 formed on a phosphor screen by the electron beams 6 landing on the peripheral portion of the phosphor screen is distorted as shown in FIG. 1B. This distortion is caused by a deflection aberration or excessive focus of the electron beams 6 in the vertical, i.e., V axis direction.
As a result, the beam spot 12 forms a halo 13A widening in the vertical direction and a core 13B extending in the horizontal, i.e., H axis direction. This deflection aberration becomes more conspicuous as the tube size is larger or the deflection angle of the tube is wider, resulting in a very low resolution at the peripheral portion of the phosphor screen.
Examples of a means for solving a decrease in resolution due to the deflection aberration are disclosed in Jpn. Pat. Appln. KOKAI Publication Nos. 61-99249, 61-250934, and 2-72546. Each of these electron gun assemblies basically has a cathode K, and first to fifth grids G1 to G5, as shown in FIG. 2A. The electron gun assembly has an electron beam generating portion GE, a quadrupole lens portion QL, and a final focusing lens portion EL sequentially arranged along the direction in which the electron beam travels.
The third grid G3 forming the quadrupole lens portion QL has three rectangular electron beam passage holes 14a, 14b, and 14c in a surface facing the fourth grid G4, as shown in FIG. 2B. The fourth grid G4 has three rectangular electron beam passage holes 15a, 15b, and 15c in a surface facing the third grid G3, as shown in FIG. 2C. The electron beam passage hole 14 (a, b, c) is formed with a shape asymmetrical to the electron beam passage hole 15 (a, b, c).
In the electron gun assembly, the lens powers of the quadrupole lens portion QL and the final focusing lens portion EL change in accordance with the beam deflection amount of the electron beam to compensate for the influence of any deflection aberration produced by the deflecting magnetic field on the electron beam deflected to the screen periphery, and to correct distortion of the beam spot on the phosphor screen.
Even with this correction means, however, when the electron beam is deflected toward the peripheral portion of the screen, generation of a halo of the beam spot can be suppressed, but vertically collapsing distortion of the beam spot cannot be satisfactorily corrected.
FIG. 3 is a view for explaining the electron beam orbit and the lens operation in the electron gun assembly shown in FIG. 2A. In FIG. 3, the solid lines represent the electron beam orbit and the lens operation when electron beams are not deflected and focus on the center of the screen. The broken lines represent the electron beam orbit and the lens operation when electron beams are deflected and focus on the peripheral portion of the screen.
As shown in FIG. 3, when electron beams are not deflected, the electron beams focus on the center of the phosphor screen by the action of only the main electron lens portion EL indicated by the solid lines. When electron beams are deflected, electron beams focus on the peripheral portion of the phosphor screen by the action of the quadrupole lens portion QL located on the cathode side of the main electron lens portion EL, the main electron lens portion EL, and a deflection yoke lens portion DYL, i.e., a deflection aberration component included in the deflecting magnetic field formed by a deflection yoke.
Since a color cathode ray tube generally has a self-convergence deflecting magnetic field, a focusing lens is formed in only the vertical direction V while the focusing force in the horizontal direction H remains unchanged. In FIG. 3, therefore, the lens action of the deflecting magnetic field in the horizontal direction H is not illustrated.
When electron beams are deflected, the lens power of the main electron lens portion EL weakens as indicated by the broken lines, and the quadrupole lens portion QL is formed as indicated by the broken lines so as to compensate the focusing action in the horizontal direction H. Consequently, electron beams pass through electron beam orbits indicated by the broken lines in FIG. 3 to focus on the peripheral portion of the screen.
In the example of FIG. 3, when electron beams are focused on the phosphor screen, the principal plane of the lens, i.e., virtual lens center (cross point between the orbit of a beam emitted by the cathode and the orbit of a beam incident on the phosphor screen) is at position A when the electron beam is not deflected. To the contrary, when the electron beam is deflected, the principal plane of the lens in the horizontal direction H moves from position A to position B on an accordance with generation of the quadrupole lens portion QL. At this time, the principal plane of the lens in the vertical direction V moves from position A to position C on the phosphor screen side.
The principal plane in the horizontal direction H therefore moves back from position A to position B on the cathode side to increase the lens magnification. This increases the beam spot diameter on the phosphor screen in the horizontal direction. The principal plane in the vertical direction V moves forward from position A to position C on the phosphor screen side to decrease the lens magnification. This decreases the beam spot diameter on the phosphor screen in the vertical direction. As a result, the magnification becomes different between the horizontal and vertical directions, and a beam spot formed by the electron beam landing on the peripheral portion of the screen is elongated in the horizontal direction H.
The present invention has been made to solve the above problems, and has as its object to provide a cathode ray tube capable of obtaining a high image quality over the entire screen by preventing distortion of the spot formed by the electron beam resulting from the difference in lens magnification between the horizontal and vertical directions when electron beams are focused on the peripheral portion of the screen.
According to the present invention, there is provided a cathode ray tube comprising:
an electron gun assembly having an electron beam forming portion for forming and emitting at least one electron beam and a main electron lens portion for accelerating and focusing the electron beam; and
a deflection yoke for generating a deflecting magnetic field for deflecting the electron beam emitted by the electron gun assembly and scanning a screen in vertical and horizontal directions,
the main electron lens portion having first, second, and third lens regions sequentially formed by a voltage distribution continuously increasing along a traveling direction of the electron beam, the second lens region having means for, assuming that horizontal and vertical directions be perpendicular to the traveling direction of a nondeflected electron beam traveling toward a center of a screen, forming an asymmetrical lens having a focusing force in the vertical direction relatively different from a focusing force in the horizontal direction.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.